Due to most electronic devices, for example printer, notebook computer, monitor, household electric appliances, and smart phone, their interior power are DC powered, therefore it is necessary to convert AC power into DC power for those electronic devices.
Enhancing power converting efficiency and power saving capability have became more important in the field of AC/DC application. Full load efficiency has always been a major consideration in designing an AC/DC power supply. However, as more and more power adapters still consume power in a standby mode, how to further improving the power saving capability at light loads and load shedding conditions has became increasingly important.
More and more AC/DC power converters support standby mode and don't need to turn off power as usual, in other words electric equipments can still consume electric power even without executing their main functions. For limiting standby time and enhancing power efficiency in the entire load ranges, various kinds of standards have been regulated and the most important one them is the energy star established by the environment protection agency (EPA) of the USA government. Energy star includes many developing standards, which can enhance the power saving capability in no load and light load conditions, increase the power efficiency in normal operation.
Flyback converter is one of the widely used power supply topologies due to low cost and less components it has.
FIG. 1 indicates the circuit of the most common used flyback converter. A household AC power inputs an input voltage through a bridge rectifier 101 and a capacitor 103 for rectifying and filtering the AC voltage into a DC voltage, then feeds the DC voltage into the input of the transformer 105. Flyback converter has a power stage including a switching transistor 107, a diode 109, a transformer 105, and an output capacitor 111. By controlling the ON or OFF state of the switching transistor 107 via a pulse width modulation (PWM) control circuit 113, a DC voltage Vout can be output through the coupled diode and capacitor on the secondary side circuit. The primary winding Np has functions for isolation, voltage conversion, and induction due to the fact that transformer 105 has an air gap. The primary winding Np receives the rectified DC voltage and stores it in the coils as the switching transistor 107 being switched ON, which transfers the magnetized energy (i.e. the DC voltage) stored in the coils into the secondary winding coils Ns as the switching transistor 107 being turned off, then stores the magnetized energy in the capacitor 111 and generates a DC voltage. An output circuit is coupled to the secondary winding Ns and a load 115 for generating the DC voltage to the load 115. A feedback circuit 117 receives the output voltage of the flyback converter and outputs the variation of voltage to the PWM control circuit 113 for controlling the switching transistor 107.
Flyback converters have been widely used for off-line power supply with power less than 100 Watts in electronic devices including printer, notebook computer, monitor, household electric equipments, smart phone and other 3C electronic products.
Normally, a flyback converter works in light load or no load condition, its output power is lower. If the flyback converter operates in a regular switching frequency, it has relatively higher portion of power loss causing the reduction of overall conversion efficiency. Generally, a switching power supply system's design parameters are decided while the specifications of a flyback converter are given. Because there are many specifications in the existing power supply systems, an optimizing power converting efficiency may not easy to reach by utilizing the same control IC to control the switching frequency curve of various flyback converters.
In order to solve the above mentioned issues, a flyback converter with an adjustable frequency curve at green mode is needed.